1. Field of the Invention
The present invention relates to an improved coal burner that reduces the formation of nitrogen oxides in pulverized coal-fired furnaces. More specifically, the present invention relates to an apparatus and method that use diffusers in connection with coal burners so that the flow of primary air and coal is split into two or more discrete streams before being discharged into the furnace. The present invention also relates to a method for forming a plurality of discrete streams leaving the primary-air/coal pipe.
2. Description of the Prior Art
Empirical studies have identified two mechanisms for the formation of nitrogen oxides, hereinafter referred to as NOx, in pulverized coal-air flames: (1) thermal reaction of nitrogen and oxygen contained within combustion air to form NOx (hereinafter thermal NOx), and (2) the oxidation of organically bound nitrogen compounds contained within coal to NOx (hereinafter fuel NOx). For conventional furnaces, thermal NOx formation becomes significant at temperatures above 2800 degrees Fahrenheit. Conversion of fuel-bound nitrogen to NOx can occur at much lower temperatures. Fuel-bound nitrogen is believed to be the source of about 75% of the total NOx resulting from combustion of pulverized coal.
The content of nitrogen by weight of coals typically burned by utilities can vary from about 0.3% to over 2.0%. A coal having 1% nitrogen by weight and a heating value of 12,000 Btu per pound would emit the equivalent of more than 0.5 pounds of NOx per million Btu's if only 20% of the fuel-bound nitrogen was converted to NOx.
Fuel NOx is produced by the oxidation of nitrogen that is present in both the volatile matter and char portions of coal. However, investigations have revealed that approximately 60 to 80 percent of the fuel NOx is produced by the oxidation of nitrogen present in the volatile matter. Volatile matter in coal is evolved during the first 200 milliseconds of combustion. Thus, by controlling the near-burner stoichiometry, fuel NOx formation rates can be reduced by creating localized oxygen deficient regions early in the combustion process when near-adiabatic conditions exist. Additionally, creating a localized oxygen deficiency at the location of the peak flame temperature reduces the formation of thermal NOx. Therefore, controlling the near burner stoichiometry is an effective method of reducing thermal and fuel NOx formation.
Slowly mixing or controlled mixing burners have been tried on wall-fired furnaces. These slow mixing burners may have two sets of registers to control secondary combustion air flow. However, two sets of registers increases the cost of producing the burners. They have had some success but are expensive and often result in increased unburned carbon in the fly ash. Excessive levels of unburned carbon in fly ash is an indication of poor combustion efficiency and is measured by determining the weight loss on ignition of fly ash. Excessive carbon in fly ash also decreases the fly ash's value as an additive for Portland cement, which is its most profitable use. Additionally, if the amount of unburned carbon in the fly ash becomes too high, then fly ash is susceptible to combustion and the resulting fires can damage power plant equipment and facilities.
These slow mixing burners may also cause flame impingement on the boiler tubes which increases corrosion and erosion of tube metal and decreases tube life. The impingement can also cause severe slagging problems. In the extreme, the flame impingement can cause tube wastage so extensive that in a matter of a few days or weeks, the furnace is forced off-line to repair or replace damages tubes. Slow mixing can also cause elevated furnace exit temperatures which can cause severe fouling. Fouling can become so bad that the unit must be taken off line to clean the heat transfer surfaces.
Slow mixing burners often do not decrease the formation of NOx as far as desired by themselves. Therefore, they are often used in conjunction with overfire air ports. The use of overfire air ports is common in the application of this technology because overfire air is a more effective NOx reduction technique than the burners. Sometimes, overfire air ports can by themselves or in conjunction with low NOx burners reduce the NOx emissions to desired levels. However, overfire air ports are expensive and can also cause many of the problems associated with the slow mixing burners. In addition, using overfire air ports can convert the entire lower part of the furnace into a fuel rich zone and create a reducing atmosphere. Reducing conditions can lead to rapid or catastrophic deterioration of boiler tube metal. This can lead to boiler tube failure in a short period of time.
Other retrofits include reburn, wherein some fuel is added to the furnace after most of the coal has burned in order to produce a fuel rich zone. After the fuel rich zone is formed, more air is injected. This technique is promising, but it is only partially developed and is known to be expensive. Also, some furnaces simply do not have sufficient furnace volume to accommodate reburn systems. In addition, reburn is most effective if the reburn fuel does not contain any fixed nitrogen. However, fuels that do not contain fixed nitrogen are usually more expensive than coal or heavy oil.
Using ammonia to react with the NOx to form molecular nitrogen has been successful in special applications. However, thermal or catalytic reduction of NOx with ammonia is expensive. Also, ammonia will react with any sulfur trioxide that is present to form a sticky solid or viscous fluid that can plug air heaters or form visible emissions. Further, any un-reacted ammonia may pass through pollution control equipment and escape into the environment. Even if an ammonia based NOx reduction system is used to obtain very low emissions it would still be desirable to reduce the formation of NOx during combustion to reduce the size and cost of any post-combustion NOx control system.
Breen, U.S. Pat. No. 4,223,615, discloses a process for placing aerodynamic spoilers that extend into the wake of the stream of primary air and pulverized coal to produce fuel-rich and air-rich zones near the burner. The technology disclosed by Breen would not be effective on burners with diffusers, because a downstream diffuser would break up the separate streams formed by the aerodynamic spoilers.
Chung, U.S. Pat. No. 5,249,535, discloses a burner tip that divides an annular pulverized coal stream into alternating fuel-rich and fuel-lean streams by means of skewed vanes that provide alternating converging and diverging channels for the primary-air and coal stream to flow through. The technology disclosed by Chung represents a complete replacement of a burner rather than a modification of an existing burner.
Rini, et al., U.S. Pat. No. 5,113,771, discloses the use of a nozzle that forms a plurality of passages for splitting the stream of primary air and pulverized coal into a corresponding plurality of separate streams. The passages are inclined and converge such that the plurality of separate streams impinge upon each other upon being discharged into the primary burn zone of the furnace.
Allen, et al., U.S. Pat. No. 4,930,430, discloses the use of guide elements and flow disturbing members arranged to deflect the flow of primary air and coal in the primary-air/coal pipe to produce regions of high fuel concentration. The action of the guide elements and flow disturbing members promotes combustion conditions that lead to low NOx emissions.
Vatsky, U.S. Pat. No. 5,347,937, discloses a plurality of angularly spaced walls within an annual passage that is part of a burner assembly for splitting up a fuel/air stream so that upon ignition of said fuel a plurality of flame patterns are formed.
Vatsky, U.S. Pat. No. 4,348,170 discloses a plurality of V-shaped members disposed within an annular passage located in a burner assembly for splitting up stream of fuel so that, upon ignition of the fuel, a plurality of flame patterns are formed.
Despite the efforts of others described above, there remains a long-felt need for an effective, relatively inexpensive retrofit scheme that reduces NOx formation in pulverized coal-fired furnaces that utilize coal burners with diffusers.